A system of that type is known from US 2001/049318.
In a normal braking situation, that known system performs progressive hydrostatic braking by gradually reducing the cubic capacity of the pump. When an emergency braking situation is detected, the cubic capacity of the pump is reduced much more rapidly, and, when said cubic capacity reaches zero, the friction brake means are actuated.
The brake system of the invention applies in particular to a working vehicle, e.g. to site plant or to a farm vehicle. In the working situation, the vehicle travels at a relatively slow working speed, generally not substantially exceeding ten kilometers per hour. For traveling between work sites, the vehicle can reach a higher speed of travel on roads.
The progressive hydrostatic braking must be effective, without however generating unacceptable discomfort for the driver of the vehicle. The braking system of a vehicle of that type must offer good performance both under working conditions, and while traveling on roads.
In addition, it is also desirable to limit the travel time of vehicles of the above-mentioned type when traveling between work sites, which makes it necessary to increase the maximum speed of on-road travel of such vehicles. Vehicles are thus designed whose maximum on-road speed can reach 40 kilometers per hour or even faster.
The brake system disclosed by US 2001/049318 makes it possible to perform emergency braking when circumstances so require. However, its normal braking, which is hydrostatic only, might not always be effective when the vehicle is in certain travel situations, in particular on the road, and when it is traveling at relatively high speeds. For example, when the engine is turning fast due to insufficient restraining torque, such normal braking might not be sufficient. Thus, it is often necessary to use emergency braking, which is uncomfortable.
The friction brake means are used for emergency braking only, after emergency hydrostatic braking has taken place. That precaution is supposed to avoid having to provide a large-size brake. Nevertheless, it is at the start of emergency braking that it is necessary to reduce the speed of the vehicle rapidly, and sudden hydrostatic braking alone is not fully satisfactory in achieving such a rapid reduction in speed. In addition, in that system, the speed of the vehicle is not taken into account for adjusting the hydrostatic braking appropriately, regardless of whether braking is normal braking or emergency braking. Therefore, that system does not make it possible to guarantee that the vehicle always stops within a distance compatible with safety requirements, without generating jolts that are incompatible with providing comfort for the driver of the vehicle.